Iron dehydroxylation

The anaerobic degradation of some hydroxybenzoates and phenols involves reductive removal of the phenolic hydroxyl group. The enzyme that dehydroxylates 4-hydroxybenzoyl-CoA in Thauera aromatica is a molybdenum-flavin-iron-sulfur protein (Breese and Fuchs 1998), and is similar to the enzyme from the nonsulfur phototroph Rhodopseudomonas palustris that carries out the same reaction (Gibson et al. 1997). 

Breese K, G Fuchs (1998) 4-hydroxybenzoyl-CoA reductase (dehydroxylating) from the denitrifying bacterium Thauera aromatica prosthetic groups, electron donor, and genes of a member of the molybdenum-flavin-iron-sulfur proteins. Eur J Biochem 251 916-923. 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

IR spectra for the pillared bentonites in the OH-stretching region show an intense and broad OH-band centered near 3640 cm this band is shifted to near 3600 cm for the ACH-nontronite sample under study, Fig. 1. After pyridine sorption, only minor changes were observed in these spectra, indicating little reaction of the hydroxyl groups present with pyridine. As the degassing temperature is increased from 200 C to 500 C, OH bands decrease in intensity due to dehydroxylation reactions of the clay lattice. Fig. 1. Dehydroxylation is more facile in the iron-containing ACH-nontronite sample. Fig. IF. 

Guseinov AA (1999) Relationship between ion conductivity and the heat-induced processes of oxidation and dehydroxylation in ferrous-magnesian micas. Geochem Int l 37 87-90 Haggstrom L, Wappling R, Annersten H (1969) Mossbauer study of iron-rich biotites. Chem Phys Lett 4 107-108... 

Thermal methods of analysis are often useful in the characterization of minerals, as described in Section 7.6.5. Aluminum hydroxides such as gibbsite show a mass loss of 34.6% on dehydroxylation thus, they show an important negative peak in DTA and a marked mass loss in TGA, and so these techniques are employed for both qualitative and quantitative characterization of these minerals. The same happens with iron hydroxides and oxohydroxides, such as goethite, lepidocrocite, and so on also, the presence of OH groups in otherwise thermally inert minerals such as hematite can be detected. 

The spectrum of celadonite can be adjusted with two ferrous doublets and one ferric doublet, the latter assigned to M2 sites (cis) [245]. A second weak Fe " doublet with relatively large quadrupole splitting (A 1.1-1.2 mm/s) (Fig. 3.31) has been ascribed to iron in dehydroxylated surface sites (DSS) [246, 247]. 

The characteristics of different mica deposits are summarized in Table 1. Muscovite is a potassium aluminium silicate which is transparent and almost colourless. It is chemically unreactive and is stable to about 600°C when dehydroxylation takes place. Phlogopite, sometimes known as magnesium mica, is generally coloured brown (but can also be green depending upon oxidation state) due to the iron which is also present... 

The above observations all indicate that substitution of iron for aluminum in the octahedral layer and of aluminum for silicon in the tetrahedral layer both cause dehydroxylation to commence at a lower temperature. This, combined with the fact that the binding energy of the hydroxyl groups also affects the results, explains the extreme complexity of the curves obtained for this subgroup. Yet all have a potential diagnostic feature in the size and configuration of the low-temperature endothermic peak. 

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